1. Field of the Invention
The field of the invention is that of the broadcasting of digital data intended to be received notably by mobile receivers in an urban environment, that is, under conditions of multiple propagation (Rayleigh Process) generating phenomena of fading, and in the presence of parasites and jamming.
The invention can be applied more particularly, but not exclusively, to the system of digital sound broadcasting known as the COFDM (Coding Orthogonal Frequency Division Multiplex) as described in the U.S. Pat. No. 4,881,241 of the present applicants.
This system of digital broadcasting is based on the combined use of a channel coding device and a method of modulation by orthogonal frequency division multiplexing.
2. Description of the Prior Art
The modulation method proper of this prior art system makes it possible to resolve the problems related to the frequency selectivity of the channel. It consists in providing for the distribution of the constituent digital elements of the data signal in the frequency time space f-t, and in simultaneously transmitting sets of digital elements on N parallel broadcasting channels by means of a multiplex of frequencies using orthogonal carriers. In particular, this type of modulation makes it possible to prevent two successive elements of the data train from being transmitted at the same frequency.
The known encoding method seeks, on the whole, to enable the processing of the samples coming from the demodulator to absorb the effect of variations in amplitude of the received signal, due to the RAYLEIGH process. The encoding is advantageously a convolutional encoding, possibly concatenated with a REED-SOLOMON type of encoding. The decoding may advantageously be a soft decision type of Viterbi decoding.
In a known way, the encoded digital elements are additionally interlaced, in time and in frequency, so as to maximize the statistical independence of the channels with regard to the Rayleigh process and to the selective character of the channel.
The demodulation of the received signal may be differential or coherent.
It is known that coherent demodulation theoretically offers greater resistance to noise than differential demodulation, making it possible to obtain a gain in performance of about 3 dB. However, it clearly appears that, under the specific reception conditions of the method, it is particularly difficult to extract a phase and amplitude reference from the modulated signal for each carrier of the multiplex. In the case of coherent demodulation, therefore, the error made in the estimation of the carrier leads to a substantial deterioration in performance characteristics. This is especially true in the case of deep and fast fading encounted when the carrier frequency or the speed of the vehicle increases.
Differential demodulation constitutes another approach, the essential usefulness of which lies in its simplicity of implementation and its absence of inertia after deep fading. It is therefore this second approach that has been used to validate the general principles of the COFDM system, at the cost of a deterioration in performance characteristics of about 3 dB as compared with coherent demodulation.